1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly, to a method for forming fine patterns in semiconductor devices.
2. Description of the Related Art
As the integration density of semiconductor devices has increased, finer patterns have been required. To meet required design rules that become gradually smaller, a variety of new manufacturing techniques are needed, including improvement of a photolithography process. For example, new light sources and photoresist materials suitable therefor are desired, as well as exposure apparatuses capable of using new light sources and photoresist materials. In addition, development of manufacturing processes appropriate for new light sources and related apparatuses to form fine patterns are further desired.
KrF, which is a light source widely used in a photolithography process, is reaching the technical limit for forming much finer patterns because KrF has a longer wavelength than the critical dimension of such patterns, which continue to decrease. Thus, studies of use of lights having shorter wavelengths have been made. As a result, techniques of using ArF gas or F2 gas as a light source have been proposed. In addition, exposure methods and apparatuses using ArF(193 nm) excimer laser or F2(157 nm) excimer laser, which is emitted from new light sources, are presently being developed.
To form fine patterns by applying the ArF excimer laser or F2 excimer laser to practical processes, it is also necessary to develop suitable resist materials and manufacturing processes. As problems with materials and techniques remain unsolved, the new light sources have not yet been applied to practical production lines. Accordingly, research for a combination of other manufacturing techniques using the existing photoresist materials (e.g., KrF resist material) has progressed along with developments in new photoresist materials and techniques that enable application of the new light sources.
A method of using a half tone phase shift mask (HT-PSM) is one technique of forming a fine contact plug using the existing KrF resist material. It is assumed that this method enables formation of a contact hole having a critical dimension of about 150 nm, even when using a KrF resist. However, in the case of using a HT-PSM, it is difficult to manufacture a mask. Also, when the density of a contact hole is high, a side-lobe may be formed. Furthermore, in the method of using HT-PSM, it is practically impossible to form hyperfine patterns such as a contact hole having a critical dimension of 100 nm or less.
Another technique of forming finer patterns using the existing photoresist is a resist flow process. According to the resist flow process, fine patterns, which are smaller than the exposure wavelength limit, can be formed even without changing resist materials or exposure apparatuses. A brief description of the resist flow process will be explained hereinafter.
A resist pattern is formed as a line and space shape or a contact hole shape such that a pattern distance is set to be more than a predetermined value. For instance, a contact hole pattern is formed using a KrF resist to have a diameter of about 180 nm. The contact hole pattern having the diameter of 180 nm can be easily embodied also by the current techniques. Next, the resist pattern is heated for a predetermined amount of time at the glass transition temperature of the photoresist material or higher. Then, flow of the resist occurs and this leads the profile of the resist pattern to inflate. As a result, a distance between patterns is decreased and the diameter of the contact hole is reduced. Based on the foregoing principle, it is feasible to form patterns having a size below the exposure wavelength limit, that is, contact hole patterns of less than 150 nm.
Even so, the resist flow process has several problems, especially when a resist pattern is heated at the high temperature, thus causing plenty of flow. For example, when plenty of flow of the resist occurs, an interfacial profile of the resist pattern may be bent like a bow. That is, a bowing phenomenon occurs. This is because a flow rate may be different according to the position of resist patterns. In other words, a lot of flow is generated at a medium portion of resist patterns while less flow occurs at lower or upper portions thereof. Thus, when a lower layer is etched by using the resist pattern as an etch mask, an etched profile may not be vertical or a desired size of pattern may not be realized due to the bowing phenomenon.
To solve the foregoing problems, a baking process may be carried out through several steps. However, this method may increase process times, thus permitting a drop in yield. Also, if the amount of flow increases, the bowing phenomenon cannot be fundamentally prevented.
Another problem of the resist flow process is that when there is any deviation in the pattern density, the amount of flow may be different depending on the deviation. For example, there may exist a deviation in the duty ratio of a contact hole according to positions due to a difference in the pattern density. Here, the duty ratio refers to the ratio of a distance between adjacent patterns to a pattern size. In the case of logic devices rather than memory devices, the deviation may be very large according to positions. When the deviation is large, if the baking process is performed, more flow occurs at a position where the distance between adjacent patterns is relatively large. As a result, patterns having the desired size cannot be formed and, more seriously, adjacent resist patterns may be connected with each other and thus damaged.
Still another problem of the resist flow process is that when it is desired to cause resist to flow more, as the amount of flow increases, it becomes more difficult to exactly control the amount of flow. In general, even finer patterns can be formed by causing much more flow. However, in the case of the resist flow process, it becomes more difficult to exactly control the desired amount of flow in proportion to the present amount of flow. Therefore, it is considerably troublesome to form uniform and fine patterns.
Consequently, conventional techniques including the resist flow process have a specific technical limit when fine patterns are formed below the exposure wavelength limit. For instance, it is possible to form a contact hole having a critical dimension of about 150 nm by using a KrF resist. But, it is not simple to form hyperfine patterns having a critical dimension of about 120 nm or less, or ultimately about 100 nm or less.